X-ray computed tomography of holographically fabricated three-dimensional photonic crystals.

The optical properties of a photonic crystal (PC) are governed by the periodicity of its lattice, geometry of its basis, and dielectric contrast of its constituents. Numerous advances in experimental fabrication and characterization of three-dimensional (3D) PCs with submicron morphology have been made since their inception, [ 1 ] along with theoretical prediction of their optical properties. [ 2 ] Their detailed morphology has typically been verifi ed directly via electron microscopy of surfaces and cross sections, [ 3‐5 ] and indirectly via optical spectroscopy and diffraction measurements, [ 6‐9 ] but not holistically by direct electromagnetic 3D tomography at length scales substantially shorter than those of optical wavelengths. Here, 3D PCs fabricated by holographic optical lithography were imaged using hard X-ray microscopy. The large penetration depth of the X-rays allowed angle-resolved two-dimensional (2D) images to be analyzed to reconstruct the full 3D morphology of a holographic PC via computed tomography (CT). The reconstructed morphology was used both as a basis for optical refl ectance spectra calculations, which compared closely to experimental measurements, and to validate a theoretical model of the PC structure. Fabrication of 3D PCs via multibeam interference lithography allows fl exible control of their basis of periodic units. [ 10‐12 ] Most theoretical proposed structures assume the basis follows